Foam packing sheet for flexographic rubber printing plates

ABSTRACT

Flexography basically is a relief printing process similar in many respects to letterpress but using fluid, fast drying inks, rubber plates, rotary presses and being commonly web fed. The rubber plates are supported on a plate cylinder usually by a fabric material carrying adhesive on its opposed major faces. Such fabric is advantageously replaced by a resilient compressible flexible composite which provides for maintaining the conformability of the usual printing plate to the work to be printed while sufficiently minimizing plate distortion as to avoid poor quality printing. The composite is compressible without significant lateral spreading and is of uniform thickness overall, suitably exhibiting a tolerance not in excess of plus or minus 0.003 inch and preferably plus or minus about 0.001 inch.

United States Patent Grupe et a1. 1 1 Sept. 9, 1975 [54} FOAM PACKING SHEET FOR 3,147,698 9/1964 ROSS 161/401 FLEXOGRAPHIC RUBBER PRINTING 3,238,086 3/1966 Tompkins 161/401 PLATES 3,338,163 8/1967 Maria Ct a]. 161/160 3,649,439 3/1972 ROSS 161/165 [75] Inventors; Edward H. Grupe, Appletom Wig; 3,705,072 12/1972 Rosvold 161/167 John F. Hechtman, Munising, Mich.

P" E. W 11" .1. V1 8'] [73] Assignee: Kimberly-Clark Corporation, \ammer 1 mm m d en 57 ABSTRACT Filed? P 1974 Flexography basically is a relief printing process simi- Appl. No.: 458,820

Related U.S. Application Data Continuation of Ser. No. 11093, Feb. 13, 1970,

abandoned.

References Cited UNITED STATES PATENTS 2/1933 Rowcll 161/401 lar in many respects to letterpress but using fluid, fast drying inks, rubber plates, rotary presses and being commonly web fed. The rubber plates are supported on a plate cylinder usually by a fabric material carrying adhesive on its opposed major faces. Such fabric is advantageously replaced by a resilient compressible flexible composite which provides for maintaining the conformability of the usual printing plate to the work to be printed while sufficiently minimizing plate distortion as to avoid poor quality printing. The composite is compressible without significant lateral spreading and is of uniform thickness overall, suitably exhibiting a tolerance not in excess of plus or minus 0.003 inch and preferably plus or minus about 0.001 inch.

4 Claims, 8 Drawing Figures WFATENTEDSEP Y 3.903194 sum 2 p 1 FIG. 4

PATENTEUSEP 1 15 3,903,794

SHEET I [1F 4 I FLEXIBLE BACKING SHEET CAST FLUID FOAM ON THE BACKING SHEET AND METER TO THICKNESS SET THE FOAM ON THE BACKING SHEET TO FORM DRY COMPOSITE ABRADE THE FOAM AS NECESSARY TO PROVIDE COMPOSITE TOLERANCE 10.003"

ADHESIVELY TREAT THE COMPOSITE ON OPPOSED MAJOR SURFACES (OPTIONAL) FIG, 6

FOAM PACKING SHEET FOR FLEXOGRAPHIC RUBBER PRINTING PLATES This is a continuationof application Ser. No. 1 1,093, filed Feb. 13, 1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improvements in arrangements for flexographic printing operations and is particularly concerned with the cushioning of the flexographic printing plates.

2. The Invention with Relation to the Prior Art Conventional printing processes have a resilient member in the printing couple. In flexographic printing the printing plates are themselves of a yieldable rubber in the form of sheets of about 105 mils thickness and such plates have the advantage of relatively low cost. However, rubber is generally incompressible, and printing pressures cause distortion of the printing surfaces, resulting in relatively poor quality printing.

Also, the rubber plate useful in the flexographic printing area is commonly produced from a matrix which is itself made from a metal plate such as a zinc or magnesium plate. While the metal plate is reasonably accurate, the matrix generally introduces a first variable and the rubber plate itself will show a second; thus, the depressed zones of the matrix which form the relief areas of the rubber plate may vary from 0.001 inch to 0.003 inch, a variation which will be reproduced in the rubber plate. Some variations may be minimized by careful plate making work but at increased plate cost. Also, though great care be taken in the plate making operation, other inaccuracies occur in plate mounting, worn equipment and the like all of which inaccuracies affect the plate compression and, consequently, also affect ultimate print quality.

The mounting of the rubber plates if normally cffected by adhesively applying a finely woven fabric of 0.015 inchor 0.020 inch to the plate cylinder and then adhesively securing the printing plate (itself about 105 mils thick) to the fabric. Such fabric usually has relatively heavy adhesive coats and adhesive cold flow may occur. Additionally, the fabric usually has a caliper variation of 0.003 inch to 0.005 inch resulting in further nonuniformities. Further, this fabric material, commonly called sticky back, is relatively expensive and substantially incompressible, that is, it will not yield significantly when pressure is applied to the printing plates. The printing plate too is pliable, has a relatively soft printing surface, and is subject to distortion when handling; also, the plate usually has a built-in distortion due to the fact that it is molded flat and then conformed to the sticky back and cylinder, causing the plate to cup providing high edges in the printing characters.

A primary object of the present invention is to provide a resilient mounting sheet composite for flexographic rubber plates which will overcome some of the inherent inaccuracies of the printing plate, will absorb some of the printing pressure, and relieve the printing plate of significant distortion but without eliminating the capacity of the rubber printing plate for intimate, ink-transferring contact while having in the system good recovery characteristics so that successive printings are of the same quality.

Another important object of the invention is to provide a resilient, compressible mounting sheet composite of controlled thickness variation overall for combination with a pliable, flexographic printing plate to minimize distortions of the plate under pressure in printing operations.

A further object of the invention is to provide a novel method of producing a resilient compressible mounting sheet for combination with a flexographic printing plate.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:

FIG. 1 is a schematic view illustrating the principal components of a flexographic printing arrangement;

FIG. 2 is a view of a plate printing cylinder, with parts broken away, illustrating a plate cylinder arrangement in accordance with this invention;

FIG. 3 is an enlarged view of a resilient compressible mounting sheet composite in accordance with the invention and adapted to provide support for the printing plates;

FIG. 4 is a view similar to that of FIG. 3 but with the mounting sheet adhesively coated;

FIGS. 5A, 5B and 5C respectively set forth a prior art arrangement and two printing plate-mounting sheet composites of this invention particularly with relation to dimensional factors; and

FIG. 6 is a flow diagram setting out a sequence of process steps in producing a preferred embodiment of the resilient compressible mounting sheet in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In essence, in the practice of the invention a resilient compressible uniform thickness mounting sheet composite is provided in cooperation with the printing plate for support of the plate; the mounting sheet is provided as a composite by (FIG. 6) casting a fluid foam on a flexible substrate, setting of the foam, grinding the foam as necessary and securing the mounting sheet with the pliable printing plate on the plate cylinder. Importantly, because of the compressibility characteristics of foam, overall thickness may be as much as plus or minus 0.003 inch. On the other hand, the grinding process is capable of producing the preferred tolerance of plus or minus 0.001 inch. It is preferred to use, as illustrated in FIG. 3 for example, a mounting sheet in which the predominant portion of the thickness is the foam with the composite having an overall thickness of about 0.040 inch. Such appears to provide adequate solidity without too rapid pressure buildup in printing press operations. As the overall thickness increases by reason of increasing foam thickness, the solidity or firmness tends to decrease and may become inadequate above about 0.060 inch, causing undesired foam movement. Below about 0.020 inch overall the pressure buildup in operation is relatively quick. A range of about 0.030 to about 0.050 inch is generally preferred. The appropriate thickness, however, may vary with the firmness of the foam structure, the greater thicknesses being more useful at the higher densities. I

Referring now more specifically to the drawings, the numeral 1 designates a fountain roll which is rubber covered and rotates in the ink fountain 2; the latter holds a fluid ink. The roll 3 is a knurled cylinder roll of metal having literally thousands of minute intaglio cells. Such cells hold a measured and controlled quantity of ink and are a main ink control factor in the system. This roll 3 serves essentially as an ink transfer roller between fountain roll 1 and the plate cylinder 4 which carries the plates designated at 5. The plate cylinder in a rotary motion prints on the web 6 which is passing between it and the backup impression roll 7.

The plate cylinder indicated at 4 is usually a metal cylinder carrying the printing plate 5. The plates are of a rubber composition, quite pliable, planar and have the capability of conforming closely with the material to be printed, that is, the web 6. Normally, such printing plates have a hardness as indicated by Shore Durometer A of about 50-55 and the plates alone deform. In the present arrangement, however, the printing plate 5 as shown in FIG. 2 is supported on the plate cylinder 4 by a resilient compressible composite mounting sheet 9 conveniently secured to the cylinder by adhesive and to the rubber printing plate by adhesive 11.

Composite mounting sheet 9 in a preferred embodiment includes a strength member 9a serving as a backing sheet to inhibit against stretch and distortion of the composite. It also includes a cushioning member in the form of a foam body 9b to provide resiliency in a plane generally perpendicular to that of the member. Suitably, strength member 9a is a bleached and scoured cotton batiste fabric of a weight of about 2 ounces per square yard and a thickness of about 8 mils.

The resilient compressible cushioning member 9b is essentially a foam having in the dry state a thickness of about 40 mils. It is characterized by a high degree of compressibility under light load without significant lat eral spreading, that is, the foam itself preferably has a Poissons ratio of less than 0.1 and the fabric further contributes to the ability of this composite to resist spreading under applied pressure, as already noted.

A useful resilient compressible composite 9 is formed by depositing on batiste cotton fabric (2 ounces per square yard and cleaned and scoured) a composition comprising a copolymer butadiene-styrene latex. The latex, of about 60% solids and in the form of a wet froth having a density of 850 grams per liter and a viscosity of 3500 centipoises, was metered to a wet thickness of about 100 mils and, after leaving the metering nip, the cloth was directed to a tenter frame and then under an infra-red gelling unit to a hot air drying and curing oven and, finally, to a windup reel. The wet foam thickness of about 100 mils provided a dry foam thickness of about 70 mils at a dry foam density of about 38 pounds per cubic foot.

The composite of 40 mil thickness indicated at 9 is obtained from the 70 mil material by grinding. In specific application, for comparative purposes, composites having overall thicknesses of 20, 40 and 60 mils were obtained. In each instance microgrinding, as discussed hereinafter, served to provide an overall thickness tol' erance for each material tested within the range of plus or minus 0.003 inch. With the cast foams of commerce it appears necessary to provide for reduction of variations as the foams, when formed, apparently usually vary about 10 mils over a small limited area of the sheet. Such is too great a variation for successful quality printing operation as achieved by this invention.

The foams useful in the practice of the invention may be controlled by density in known manner by the introduction to the latex of controlled amounts of air to limit the wet froth density. Also, the foams may be of open or closed cell. Foams derived from natural rubber, nitrile rubber, chloroprene, and blends particularly of styrene-butadiene and natural rubber, are useful. Similarly, foams prepared in situ by chemical blowing agent action, such as polyether urethanes and the polyester urethanes are suitable. However, in each instance overall thickness variations are required to be controlled to within the limits indicated.

The specific formulation of a latex for the purpose is not critical and the foams may be formed in known manner to provide the desired compressibility and resiliency. Basically, we have found that the abovementioned foam formulation containing on a dry basis about parts of butadiene-styrene (26% styrene) with usual compounding components including gelling agents and a quantity of a filler such as aluminum hydrate serves the purpose very well. Further, such types of composition may be controlled as to both wet and dry density by varying degree of frothing by controlled air introduction, for example, Thus, the froth density for the composition discussed above has been controlled between about 600 grams per liter and 950 grams per liter with corresponding density differences in the dry foam. r

The foam may be conveniently prepared to have a density in the range of between about 25 to about 45 pounds per cubic foot and preferably 30 to 40 pounds per cubic foot. For a given composition compressibility, along with other factors, will be related to foam density, that is, a particular composition will be depressed an amount which is proportional to the applied pressure and, empirically, it has been found that the following formula is useful in predicting the suitability of foams for the present purpose:

P Kx

Where x is the vertical displacement on compression in mils for an initial 100 mil thick foam, P is the applied pressure in pounds per square inch (within the limits of 3 to 8 psi. and K has a value of 0.27 to 1.50. The preferred range ofK is 0.75 to 1.25. Additionally, the useful foams will have a high recovery value, that is, a foam of 100 mils thickness will recover to about 99 to 99.5 mils thickness after repeated printing cycles. Also, foams in the mentioned density range, particularly when lightly loaded, are relatively strong, resistant to splitting and intimately unite with the backing sheets.

Micro-grinding the foam side of the fabric-foam composite is suitably carried out as described in US. Pat. No. 3,428,517 issued Feb. 18, 1969, Grupe and Hechtman inventors, and assigned to the same assignee as the present invention. The variation in thickness of the backing material should be held to a minimum for optimum printing results but, generally speaking, thickness variations within a particular backing of between about plus or minus 3 mils are suitable, but plus or minus 1 mill is preferable. The foams are themselves firm to the touch, being of relatively high density, and the sanding operation provides a surface which is smooth to the touch of a finger. Some slight depressions faintly visible to the unaided eye may exist in the foam surface but are not detrimental.

The adhesive layers 10 and 11 are preferably quite thin relative to the foam and backing strength member. Preferably, they are as indicated in FIG. 4 applied to the composite and provided on release paper 12 by the composite manufacturer. Alternatively, the adhesive may, of course, be applied at the time of mounting the composite on the printing plate. Also, the pressure sensitive material may be either coated on the composite or may be applied as a film at time of mounting on the printing plate. Usual pressure sensitive adhesives having a natural rubber or synthetic rubber base are considered useful, particularly when applied at the time of sheet mounting; the tack of such adhesives may be enhanced as is known by the inclusion of resin tackifiers. Certain synthetics such as the polyacrylates inherently have suitable adhesive properties. Snythetic polymers based on the higher acrylate ester monomers, for exam ple, acrylate esters of butyl alcohols or its higher homologues are effective. Very suitably, an adhesive such as Hycar 2600 X146 manufactured by B. F. Goodrich Chemical Company, Akron, Ohio, is very suitable. Such adhesive is advantageous in that it intimately coats the foam and fabric filling in any slight irregularities and absorbing any dust occurring on the foam surface.

It is important to commercial practice that the arrangement of printing plate and compressible material be useful with conventional printing press structures. To this end beneficial modifications may be made in the usual printing plate dimensions. As illustrated in FIG. 5A, the usual prior art printing plate has an overall thickness dimension A" of about 105 mils and a supporting mounting sheet thickness 8" of about mils giving an overall thickness of 125 mils. The plate itself will commonly have a type height of C of about 40 mils and a body thickness D of about 65 mils.

The present invention provides for a reduction in overall plate thickness, thus conserving the high quality rubber gum employed in the masking of such plates and minimizing the distortion involved in accommodating flat plates to the curvature of the plate cylinder. Thus, the new arrangement may suitably have a mounting sheet thickness B (FIGS. 58, 5C) of about 40 mils and a plate thickness A of 85 mils with 40 mils constituting the type height C and 45 mils in the plate body D. Thus, the printing plate itslef may be 20 to 40 mils less thick than the conventional commercial plate. In general, the mounting sheet may be to 90% of the thickness of the printing plate itself.

The printing plate, as already noted, is pliable, stretchable and subject to being deformed in the manual handling accompanying mounting. Such mounting may be effected in a number of ways. In one method the adhesive carrying composite may be first mounted on the print roll and then the print plate may be positioned on it.

Alternatively, the composite may be applied to the print plate in the flat condition and this assembly may be mounted on the print roll.

In general, it is preferred that the strength member 9a be adhesively provided against the printing plate (FIG. 5B) as such aids plate handling by providing support to the plate as the plate is manipulated into position on the print roll. It should be appreciated that this manipulation of the plate is a quite sensitive one and that the advantages of support is quite real.

Various modifications may be made in specific as pects of the embodiments of the invention. For example, it is particularly contemplated that a variety of substrates such as synthetic fabrics of low stretch characteristics, impregnated papers and the like may serve as the strength member.

Additionally, it should be noted that the utility of the resilient compressible mounting is particularly apparent in printing half tone screens on a daily basis, filling in of the tiny spaces between relief half tone dots being largely avoided by elimination of the dot distortion of the rubber plate. Such ability, in addition to an enhanced reproduction capacity of the usual line and solids work, particularly characterizes the invention.

We claim:

1. A support for a pliable printing plate consisting of a composite of a strength member in the form of a flexible backing sheet and a cushioning member of a resilient foam which is of greater thickness than the said flexible backing sheet intimately united with the said sheet on one surface of the sheet and freely flexible with the sheet as a unit, said foam having a Poissons ratio of less than 0.1 and a density of between about 25 and 45 pounds per cubic foot and having an overall thickness of between about 0.020 inches and 0.060 inches and a thickness tolerance of between about plus and minus 0.003 inch, the said foam on the surface remote from the backing sheet being smooth and firm to the touch.

2. In combination, an arcuate flexographic printing plate which is pliable and has a relatively soft, stretchable printing surface subject to being deformed, and a flexible mounting sheet support for the printing plate adhered to the concave surface of the arcuate plate, said support comprising a flexible strength member and a foam cushion member intimately adhered to the strength member and being limitedly compressible with said printing plate without being subject to significant lateral spreading to absorb pressure applied to said printing plate, said foam cushion member being of greater thickness than said strength member, said composite being smooth surfaced to the touch on the side remote from the strength member a d the thickness of the mounting sheet being between about 25 to of the overall thickness of the printing plate itself, the foam cushion member having a density in the range of about 25 to 45 pounds per cubic foot.

3. The combination according to claim 2 in which the strength member of the flexible support abuts the printing plate.

4. The combination according to claim 2 in which the cushion member of the flexible support abuts the printing plate.

l I l 

1. A support for a pliable printing plate consisting of a composite of a strength member in the form of a flexible backing sheet and a cushioning member of a resilient foam which is of greater thickness than the said flexible backing sheet intimately united with the said sheet on one surface of the sheet and freely flexible with the sheet as a unit, said foam having a Poisson''s ratio of less than 0.1 and a density of between about 25 and 45 pounds per cubic foot and having an overall thickness of between about 0.020 inches and 0.060 inches and a thickness tolerance of between about plus and minus 0.003 inch, the said foam on the surface remote from the backing sheet being smooth and firm to the touch.
 2. In combination, an arcuate flexographic printing plate which is pliable and has a relatively soft, stretchable printing surface subject to being deformed, and a flexible mounting sheet support for the printing plate adhered to the concave surface of the arcuate plate, said support comprising a flexible strength member and a foam cushion member intimately adhered to the strength member and being limitedly compressible with said printing plate without being subject to significant lateral spreading to absorb pressure applied to said printing plate, said foam cushion member being of greater thickness than said strength member, said composite being smooth surfaced to the touch on the side remote from the strength member and the thickness of the mounting sheet being between about 25 to 90% of the overall thickness of the printing plate itself, the foam cushion member having a density in the range of about 25 to 45 pounds per cubic foot.
 3. The combination according to claim 2 in which the strength member of the flexible support abuts the printing plate.
 4. The combination according to claim 2 in which the cushion member of the flexible support abuts the printing plate. 